1. Field of the Invention
The present invention relates especially to an imaging device which is capable of correcting a deviation of an imaging plane due to a temperature change from a focal plane on which an object is in focus.
2. Description of Related Art
Such imaging devices as a monitoring camera and a camcorder operate on an auto-focus control mode (First Mode) and a manual control mode (Second Mode). There are a couple of manual control modes, a manual-focus control mode and a manual-focus trace control mode.
A focus lens is automatically moved to a focal lens position with which an object is in focus on the imaging plane when the imaging device is on the auto-focus control mode. The focus lens is manually moved by an operator to the focal position when the imaging device is on the manual-focus control mode. The focus lens is automatically moved to have the object continually imaged in focus on an imaging plane as a zoom lens is manually moved by the operator, when the imaging device is on the manual-focus trace mode.
It is necessary to move the focus lens to an accurate focal position with which the object is imaged in focus on the zoom lens being moved, when the imaging device is on the manual-focus trace mode. For this purpose, focal lens position trajectory data, which indicates the focus lens position with respect to the zoom lens position, are used in the manual-focus trace control mode.
With reference to FIG. 4, the focal lens position trajectory data are explained. FIG. 4 shows a graph indicating the focal lens position trajectory data. The horizontal axis of FIG. 4 indicates the zoom lens position while the vertical axis indicates the focal position of the focus lens. This graph indicates the focal positions of the focus lens for a constant object distance with the zoom lens position varying. A plurality of graphs corresponding to various object distances are shown for the zoom lens position on the Tele side. Every time the operator moves the zoom lens, the focus lens is automatically moved to the focal position suited for the zoom lens position according to these graphs.
On the other hand, if the temperature of the imaging device changes, a focal distance of the imaging device could change significantly due to the physical property change of a plastic material etc. used. In order to prevent the object from being imaged out of focus on the imaging plane due to the focal distance change, temperature compensation data, which are obtained in advance by measurement or calculation, are used.
FIG. 5 is a graph indicating an example of the temperature compensation data. The horizontal axis of FIG. 5 indicates the zoom lens position while the vertical axis of FIG. 5 indicates the correction distance on the focus lens position. The correction distance on the focus lens position varies between the lenses and depends on the temperature. Correcting the focal lens position trajectory data on the basis of these temperature compensation data, the focus lens can be positioned accurately at a focal position in spite of the temperature change.
JP11-142714A discloses a control method to keep an object focused on an identical plane by obtaining the correction distance on the focus lens position making use of the temperature compensation factor which is measured or calculated in advance.
However the control method disclosed by JP11-142714A utilizes temperature compensation data obtained by measurement or calculation in advance and the focus lens position obtained based on the temperature compensation data is not necessarily optimum for any of focus lenses mass-produced. Therefore there could be a possibility that the accuracy on focusing lowers.
The present invention is intended to provide an imaging device with which the focus lens is always positioned to an accurate focal position with which an object is imaged in focus, even when the temperature of the imaging device is changing.